E. F. S. Termond

Decreased plasma albumin concentration results in increased volume of distribution and decreased elimination of midazolam in intensive care patients

The pharmacokinetic parameters of 16 patients in the intensive care unit, sedated with midazolam, were evaluated. A large variation was observed in the plasma concentration of midazolam and between the plasma concentration of midazolam and its metabolite 1‐hydroxymethylmidazolam glucuronide. The plasma albumin concentration governs the volume of distribution of midazolam. Decreased plasma albumin concentration (25 gm/L) results in an increased volume of distribution and a decreased elimination rate of midazolam. The observed plasma concentration ratio between the parent drug and its metabolite 1‐hydroxymethylmidazolam glucuronide is governed by the variables of protein binding, the metabolic rate of midazolam, and the renal clearance of the glucuronide metabolite itself (which can be considered as a measure of the kidney function of the patient).

Plasma and urinary excretion kinetics of oral baclofen in healthy subjects

SummaryBaclofen, a centrally acting muscle relaxant, is used in the treatment of spasticity. Its pharmacokinetics has been derived from plasma and urine data in four healthy subjects, whose renal function was simultaneously measured.After oral administration of a single 40 mg dose, baclofen was mainly excreted unchanged by the kidney, 69 (14) %. The half-life, calculated from extended least squares modelling (ELSMOS) both of plasma and urine data was 6.80 (0.68) h, which is longer than reported in most studies based solely on plasma data.The renal excretion rate constant had the high mean value of 0.35 (0.24) h−1, and the apparent renal clearance of baclofen equalled the creatinine clearance. Passive tubular reabsorption is relatively unimportant, since no dependence was observed on variables urine flow or pH.Although active tubular secretion may contribute to its renal clearance, as shown by the effect of coadministration of probenecid, glomerular filtration appears to be the dominant transport mechanism.

Dosage adjustment for ceftazidime in patients with impaired renal function

SummaryCeftazidime has good antibacterial activity against many Gram-negative micro-organisms including Ps. aeruginosa.The aim of the present study was to calculate a dosage adjustment regimen for renal failure patients and to test it in a second group of patients. A study was made of the pharmacokinetics of ceftazidime 1 g given as a single bolus i.v. injection in 20 patients in an intensive care unit with varying degrees of renal function, including patients on regular haemodialysis. The serum half-life of elimination (t1/2β) varied from 1.6 to 45 h depending on renal function. During haemodialysis the mean t1/2 was 4.7 h. A good correlation between the renal clearance of creatinine and ceftazidime was observed. In most patients protein binding was lower than previously observed. From the pharmacokinetic data, a dosage adjustment regimen for patients with renal insufficiency was calculated, which studies in 7 further patients showed to be effective.

Effects of methoxy groups in the NI-substituent of sulfonamides on the pathways of elimination in man. The acetylation-deacetylation equilibrium and mechanisms of renal excretion of sulfisomidine, sulfamethomidine and sulfadimethoxine.

Sulfisomidine, sulfamethomidine, sulfadimethoxine and their corresponding N4-acetyl derivatives were administered to man. The percentages of acetylation and deacetylation and those of protein binding, the half-lives of elimination and the apparent and true renal clearance values were measured. No acetylation phenotype could be demonstrated in these compounds. Methoxy substitution in the N1-pyrimidine group of sulfisomidine affects predominantly the renal clearance value and mechanism of the parent compound but has no influence on the renal clearance of the N4-acetyl derivatives.The renal clearance value of sulfisomidine is 232±33 ml/min, of sulfamethomidine 21.60±16.4 ml/min and of sulfadimethoxine 10.87±10.44 ml/min. The renal clearance values of the corresponding N4-acetylsulfonamide derivatives are 314±91 ml/min, 342±63 ml/min and 202±65 ml/min respectively.Tubular reabsorption, caused by methoxy substitution in the N1-pyrimidine ring, lowers the rate of elimination and increases the half-life. The half-life of sulfisomidine is 8.5±0.5 h, of sulfamethomidine 27.8±5.3 h and of sulfadimethoxine 34.6±10.4 h.

The effect of the molecular structure of closely related N1-substituents of sulfonamides on the pathways of elimination in man

Sulfadiazine, sulfamerazine, sulfadimidine and their corresponding N4-acetyl derivatives were administered to man. The percentages of acetylation and deacetylation, protein binding, half-lives of elimination and apparent and true renal clearance values were measured. Methyl substitution in the N1-pyrimidine ring favours acetylation by an additional N-acetyltransferase isoenzyme present in ‘fast’ acetylators only. Methyl substitution in the N1-pyrimidine ring favours renal clearance of the N4-acetylsulfonamide derivatives. The N1-substituent probably reinforces the binding of the N4-acetyl group to the active tubular transport mechanism. The renal clearance of these sulfonamides is not dependent on the structure of the N1-substituent.

Some Pitfalls in Selecting Descriptive Pharmacokinetic Models

Some pitfalls in selecting pharmacokinetic models are enumerated. To calculate the pharmacokinetic parameters of a drug that exhibits a biphasic convex plasma concentration-time curve, a two-compartment model does not automatically have to be applied. When only the parent drug in plasma is considered, a two-compartment model seems to be most appropriate. However, when the kinetic behavior of the metabolite has to be taken into account, and when a metabolic equilibrium underlies the metabolic elimination, the two-compartment model may not be appropriate. Also, when calculating the kinetic parameters of a drug with a concave biphasic plasma concentration-time curve, a capacity-limited metabolic conversion is not the automatic explanation for this observation. Limitations in renal excretion and bioavailability may be the reasons for this behavior. Convex and concave biphasic plasma concentration-time curves are illustrated, using sulfonamides as test compounds.

Comparison of linear and tapered intravenous infusion of methotrexate in oncochemotherapy. A theoretical approach

In oncochemotherapy with methotrexate (MTX) a peripheral concentration >0.45 mg/l and a plasma concentration <45 mg/l must be maintained for 20 h. The time periods required to reach and maintain steady-state concentrations after tapered and linear intravenous infusion were compared. Pharmacokinetic analyses according to a two-compartment model were used to calculate dosage regimens and concentration profiles by means of the Bayesian General Modelling Program (BM) and NONLIN. When the dosage regimen is based on a steady-state concentration in the peripheral compartment (which is the target compartment for MTX) tapered infusion reaches this concentration 40% faster and maintains it 12.5% longer, but no difference is found if the dosage regimen is based on a steady-state concentration in the central compartment. In theory the two-step 24-hour tapered infusion can be replaced by a bolus injection plus linear infusion in the ratio 1∶2 of the total dose. These dosage regimens are to be preferred over linear infusion.